Rethink "not created here" save failure

[cmyr]

currently we categorically refuse to save any file that we did not create, with the rationale that our save operation is going to be lossy; there are parts of the UFO that we do not currently bother loading, and we only save what we've loaded.

This seems a bit zealous, though; it would make more sense to just refuse to save over a file we didn't create, but we should be okay with letting people do "save as" and creating a new file.

(see #178)